1. Field of the Invention
The present invention relates to telecommunications systems. More particularly, the present invention relates to a system and method for providing an application of a monitoring access to a Digital Subscriber Loop (DSL) circuit, so that the monitor access to the circuit is completed without disrupting the ongoing transmissions within the DSL circuit. The establishment of such a monitor access without disruption is referred to as a “hitless” access. The monitoring access application according to exemplary embodiments of the present invention is referred to as the Hitless Monitor Access (HMA) technique.
2. Background Information
Public telecommunications systems include subscribers who are coupled to a telecommunications network with a twisted pair wire loop, commonly known as a subscriber loop. Digital transmission systems based on local subscriber loops are generally called Digital Subscriber Loops (DSLs). Line codes of various formats are used to convey digital data over existing twisted-pair copper telephone lines connecting the telephone company Central Office (CO) to subscribers. Conventional DSL data modems are designed to provide service to a certain percentage of customers at a prescribed data rate. In general, telephone lines employ twisted pairs of wire in order to mitigate crosstalk that can occur between tightly packed pairs carrying unrelated information streams.
Although there are several DSL variants (referred to collectively as “xDSL”), one DSL format is called Asymmetrical Digital Subscriber Loop (ADSL). ADSL has been defined by standards bodies as a communications system specification that provides highly asymmetrical data rates—a low-rate data stream from the subscriber to the CO (from 32 kbps to approximately 640 Kbps when sending data, referred to as the “upstream” rate), and a high-rate data stream from the telephone company CO to the subscriber (from 32 kbps to approximately 6 Mbps when receiving data, referred to as the “downstream” rate) over the same single pair. ADSL can use either Carrierless Amplitude Phase (CAP) modulation or Discrete Multi-Tone (DMT) modulation. In general, in present implementations of the DMT modulation technique, the two directions of information flow are disjoint in the frequency domain. Consequently, it is comparatively simple to protect the receiving means at each end of the path from the co-located transmitting means.
ADSL modems can be installed in pairs, with one of the modems installed at the subscriber location, known as Customer Premises Equipment (CPE), and the other in the telephone company's CO servicing that subscriber. The pair of ADSL modems are connected to the opposite ends of the same twisted-pair and each modem can only communicate with the modem at the other end of the twisted pair. The CO will have a direct connection from its ADSL modem to the service provided (e.g., movies, Internet, etc.). In general, an ADSL modem operates at frequencies higher than the voice-band frequencies.
Another high speed data service is known as a Symmetric Digital Subscriber Loop (SDSL), wherein, unlike an ADSL service, the information rate is intended to be equal in both the upstream and downstream directions. Currently, data speeds as high as 1.5 Mbps in each direction are common, again employing only a single pair of wires. The operating range of a SDSL circuit is, however, limited to approximately 10,000 feet. Furthermore, with current SDSL techniques, the send and receive frequency spectra completely overlap. Thus, the receiver at each end must not only equalize for channel dispersion introduced by the twisted pair, but must also discriminate against the co-located transmitter signal through a process known as echo cancellation. Because of the fast data rates desired, the available bandwidth of the twisted pair must be vigorously exploited.
Inevitably, DSL transmission through a twisted wire pair introduces distortions, and is limited by such things as, for example, loop loss, the noise environment, and modem transceiver technology. The impairments that must be tolerated increase with loop length and bandwidth employed.
For the purposes of network maintenance and assurance of quality of service, it is necessary for the provider of network services to be able to monitor the path established between connected users at various points throughout the network. One such point is the loop connecting the serving modem to the subscriber modem. Abrupt connection, without disruption, of monitoring means to the loop, where the transmission methods are analog, can be conventionally accomplished when narrow bandwidth services are transported, such as Plain Old Telephone Service (POTS) or comparatively slow voice-band modem service. Even well-established high speed data services such as T1, which operates at 1.544 Mbps unidirectionally on any one pair, are simple to monitor, or sample, at an analog point. This is a result of, for example, narrow operating bandwidths, rudimentary encoding techniques, short distances that result in only modest signal level losses, and unidirectional transmissions.
In an attempt to increase the utilization of presently-installed twisted pair loops by employing bi-directional information flow, faster data rates, and longer distances between regeneration devices, DSL services have adopted modems employing considerably more complicated encoding techniques, and significantly more complicated receiving means. One negative ramification of this is that the permissible degree to which a selectively-applied monitor device can alter the apparent characteristics of a transmission path without disruption is markedly reduced.
In order to ensure quality of service, it is often necessary to unobtrusively monitor the progress of communications over the transmission media by connection to the media itself. Unless the monitor facility is permanently in place, the introduction or removal of the monitor, to some degree, disturbs the transmission parameters of the media. In order to minimize disruption of information flow, the loading of the transiently-applied monitor device can be reduced as much a possible, to the point that it can be abruptly applied without harm. However, practical limits prevent the economic realization of a shared monitor with sufficiently large bandwidth and low internal noise, simultaneously with sufficiently slight loading, to consistently permit abrupt application upon, or removal from, presently-employed conventional circuits.
Further, although monitor access can be performed in conventional telephone data transmission systems, as described, for example, in U.S. Pat. No. 6,453,014, it has been difficult to obtain, without disruption, in DSLs.
For example, to obtain, at the point of access, a sample of the voltage waveform appearing between the tip and ring lead of the selected loop, without affecting any of the twisted pair transmission characteristics or disrupting the ongoing communications, a near infinite input impedance buffer amplifier can be employed with accompanying means, of vanishingly-small physical dimensions, to select and connect to the target loop. Alternatively, a less-than-ideal voltage monitor device could be permanently attached to each loop that could be potentially selected. Unfortunately, neither case is practical.
In contrast, a directional coupler can also be used as a sampling device, with the added benefit that the energy contribution of each modem can be sampled and substantially separated. This directivity is a predictable consequence of the characteristics of transmission lines which have been intentionally coupled by placement in close proximity to one another, over a length significant relative to the wavelength of the lowest frequency of interest. However, for the DSL frequency band of approximately 30 kHz to 1 MHz, the dimensions of a true coupled line directional coupler would be ponderous. Thus, an approximation which would be more compact is desired.
As one of ordinary skill in the art would recognize, such an approximation to a distributed directional coupler can be constructed using lumped circuit elements (e.g., discrete resistors, capacitors, and inductors), within any arbitrary accuracy over a given bandwidth, as a function of how finely the constituent inductive and capacitive elements are divided. This approach, however, also poses practical difficulties. Specifically, a lumped-element directional coupler involves many more parts than a simple bridging monitor, some of which (e.g., at least one impedance element) must be introduced in series with the tip and ring leads, not simply tapped to them. The abrupt insertion or removal of such a coupler would create severe data circuit disruptions.